Australia announced Monday that it was extending working holiday visas to allow young travellers to stay longer in the country to help meet a shortage of farm labourers.

The change allows travellers on so-called "backpacker visas" to remain in Australia for up to three years if they spend at least six months doing agricultural work.

Previously the one-year "Working Holiday Maker" visas allowed travellers to remain for a second year if they took up work in the remote Northern Territory.

From July 2019, they can extend this to a third year as long as they spend six months working in agricultural regions suffering from particularly acute labour shortages.

The new rules were announced by Prime Minister Scott Morrison during a visit to farming communities in the eastern state of Queensland, a key battleground for his fragile coalition government which must face a national election by May.

Australia's conservative government has since 2017 been reducing the scope of temporary working visas as part of a broader effort to curb immigration.

But the agriculture sector has complained of severe labour shortages during harvest periods, especially in rural Queensland, prompting Monday's changes.

More than 200,000 working holiday maker visas were granted in 2017-18, with Britain, Germany and France providing the most participants from the 45 nations eligible for the programme.

Last week a survey published by the University of New South Wales found that most international students and backpackers working in Australia earned only a fraction of the minimum wage.

"Our study confirms that Australia has a large, silent underclass of underpaid migrant workers," said UNSW lecturer Bassina Farbenblum. "The scale of unclaimed wages is likely well over a billion dollars."